Use drm_get_format_name to print more readable pixel format names
in debug output.

Also unify the debug messages to say "unsupported pixel format",
which better describes what is going on.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>